Honouring Sadie

We're expecting baby #4 in August, it'll be our 2nd daughter & last child. I'd really like to use my Nanny's name as she passed away last year, her name was Sarah, but she was known as Sadie.

I don't want to use it as a first name in its current form because I'm not big on diminutives as given names, and prefer longer names which can be shortened. Sarah isn't an option either as it's my sister in laws name (although its a middle name option). So would like to either use it as a middle or find a longer form of Sadie.

We live near London and I prefer classic but underused names, nothing with crazy spellings or ending in 'lyn' or 'ley'.
Our surname begins with a W, 3 syllables and is the same as Mrs Doubtfire's